Tuning the Properties of Donor–Acceptor and Acceptor–Donor–Acceptor Boron Difluoride Hydrazones via Extended π-Conjugation

Molecular materials with π-conjugated donor–acceptor (D–A) and acceptor–donor–acceptor (A–D–A) electronic structures have received significant attention due to their usage in organic photovoltaic materials, in organic light-emitting diodes, and as biological imaging agents. Boron-containing molecular materials have been explored as electron-accepting units in compounds with D–A and A–D–A properties as they often exhibit unique and tunable optoelectronic and redox properties. Here, we utilize Stille cross-coupling chemistry to prepare a series of compounds with boron difluoride hydrazones (BODIHYs) as acceptors and benzene, thiophene, or 9,9-dihexylfluorene as donors. BODIHYs with D–A and A–D–A properties exhibited multiple reversible redox waves, solid-state emission with photoluminescence quantum yields up to 10%, and aggregation-induced emission (AIE). Optical band gaps (or highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps) determined for these compounds (2.02–2.25 eV) agree well with those determined from cyclic voltammetry experiments (2.05–2.42 eV). The optoelectronic properties described herein are rationalized with density functional theory calculations that support the interpretation of the experimental findings. This work provides a foundation of understanding that will allow for the consideration of D–A and A–D–A BODIHYs to be incorporated into applications (e.g., organic electronics) where fine-tuning of band gaps is required.


■ INTRODUCTION
Donor−acceptor (D−A) and acceptor−donor−acceptor (A− D−A) molecules that benefit from extended π-conjugation have been widely studied as a result of their unique properties such as charge transfer, 1−10 thermally activated delayed fluorescence, 5,11−18 and aggregation-induced emission (AIE) (Figure 1). 19−26 This has led to their exploitation in a number of applications including organic light-emitting diodes, 26−29 organic photovoltaic materials, 30−47 and bioimaging. 13,48−52 In order for molecular materials to be utilized in the aforementioned applications, control over the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies is often required. Incorporating heteroatoms (e.g., N, O, S, B) into π-conjugated systems is one potential avenue for the alteration of optoelectronic properties, such as the band gap (E g ). When 1, which contains several thiophene (TH) groups appended to a fluorenone core, was incorporated into bulk heterojunction solar cells the power conversion efficiency (PCE) reached up to 0.8%. 53 When 2 was incorporated into an organic solar cell, a PCE of 18% was achieved, which is the highest PCE reported to date for solar cells employing nonfullerene acceptors. 54 Alternatively, electron-deficient boron-containing compounds have been integrated into D−A and A−D−A compounds as the electron acceptor component. Heterocycles based on boron difluoride (BF 2 ) adducts of chelating N-donor ligands are molecular materials that offer useful and potentially tunable redox, absorption, and emission properties. Perhaps the most well-known of these materials are the boron dipyrromethenes (BODIPYs), which have been highly sought after due to their narrow absorption and emission profiles, very high emission quantum yields (Φ em ) in solution, and rich redox chemistry. 55 −57 BODIPYs have been incorporated into molecular materials with an A−D−A architecture (e.g., 3) and exhibit near-infrared absorption and emission, electrochemical band gaps (E g CV ) as low as 1.67 eV, and the ability to generate singlet oxygen. 58 Combining TH donors with BF 2 -containing materials, such as BODIPYs, has resulted in red-shifted emission wavelengths, sometimes into the near-infrared region, due to the expansion of the π-electron system. 59 Our group has recently demonstrated that when BF 2 formazanates are bridged by a strong donor such as benzodithiophene (4), an E g CV of 1.21 eV and an optical band gap (E g Opt ) of 1.38 eV can be achieved. 60 The purpose of this work was to incorporate boron difluoride hydrazones (BODIHYs) as the electron acceptors in D−A and A−D−A scaffolds to demonstrate the ease of property tuning for these unique molecular materials via extended π-conjugation. BODIHYs (e.g., 5) are BF 2 -containing heterocycles based on chelating pyridylhydrazone ligands, and these materials exhibit aggregation-induced emission and interesting redox chemistry. 61−68 We recently developed triphenylamine−BODIHY conjugates (e.g., 6) that exhibit dual emission, AIE, multiple redox waves, and charge-transfer  characteristics. 61 Inspired by the traits associated with these BODIHY-bearing strong triarylamine donors, we were interested in utilizing BODIHYs as electron acceptors and altering the nature of the donor in D−A and A−D−A architectures. Herein, we present a systematic approach to investigating structure−property relationships for BODIHYs linked to several donors in D−A and A−D−A molecular frameworks and present their optoelectronic, AIE, and redox properties.
■ RESULTS AND DISCUSSION Synthesis. We utilized Stille cross-coupling reactions for the production of D−A and A−D−A BODIHYs by modifying a procedure used to prepare a related family of compounds (Scheme 1). 60 Our approach began by preparing Br−BODIHY 7 according to a literature procedure. 61 Benzene (BE), thiophene (TH), and 9,9-dihexylfluorene (FL) were chosen as donors due to the commercial availability and/or synthetic accessibility of the relevant synthons (Figures S1 and S2) and the variation of the size of the corresponding π-electron systems. BODIHY 7 was combined with monosubstituted organotin reagents of BE, TH, or FL using 5 mol % Pd 2 (dba) 3 / 10 mol % P(o-tolyl) 3 in toluene and heated to 170°C with stirring for 15 min in a sealed tube (pressure 3.2−4.3 bar) to yield BODIHYs 8−10 ( Figures S3−S8). BODIHYs 11−13 were prepared similarly using disubstituted organotin reagents of BE, TH, and FL (Figures S9−S14). To purify BODIHYs 8− 13, we used column chromatography (silica gel; 3:1 CH 2 Cl 2 / hexane), which resulted in their isolation in yields ranging from 23 to 81%. Despite several attempts, we were unable to grow single crystals of the title compounds, which precluded X-ray crystallography studies. To probe the thermal stability of compounds 8−10, 12, and 13, thermal gravimetric analysis experiments were conducted, and it was found that the title compounds were thermally stable (i.e., retained 98% of their mass) up to a minimum of 190°C with thermal stability windows extending to maximum values of nearly 270°C for select compounds ( Figure S15).
Absorption and Emission Properties. The absorption and emission properties of BODIHYs 8−13 were explored experimentally using ultraviolet−visible (UV−vis) spectroscopy ( Figure 2 and Table 1) and will be discussed in comparison to the properties of BODIHY 5c, which possesses phenyl substituents. 62 The wavelengths of maximum absorption (λ max ) of BODIHYs with extended π-electron systems were redshifted relative to 5c (λ max = 437 nm in tetrahydrofuran (THF)): Δλ max = 13 nm for 8, Δλ max = 19 nm for 9, Δλ max = 20 nm for 10, Δλ max = 31 nm for 11, Δλ max = 50 nm for 12, and Δλ max = 29 nm for 13. The spectra collected in both CH 2 Cl 2 and toluene revealed similar trends. The λ max values determined experimentally agreed well with those calculated using time-dependent DFT (TDDFT) ( Table S1). Solvatochromism and a decrease in Φ em were observed upon increasing the polarity of the solvent (i.e., toluene → THF), supporting the hypothesis that the relevant electronic transitions observed for BODIHYs 11−13 may have chargetransfer character. Like other BODIHYs, 61−63,67,68 compounds 8−13 exhibited weak solution-state emission due to nonradiative relaxation pathways arising from the rotation and/or vibration of the aryl substituents appended to the BODIHY heterocycle. The wavelength of maximum emission (λ em ) for BODIHY 5c in THF is 547 nm and λ em for 8−13 were red- shifted by Δλ em = 2 nm for 8, Δλ em = 29 nm for 9, Δλ em = 57 nm for 10, Δλ em = 48 nm for 11, Δλ em = 96 nm for 12, and Δλ em = 45 nm for 13, with Φ em < 1% for all compounds. To explore the solid-state properties of BODIHYs 8−13, we prepared thin films by spin-coating saturated CH 2 Cl 2 solutions of each BODIHY onto quartz slides and measured the corresponding absorption and emission spectra. The absorption and emission bands observed for the thin films were broadened, λ max and λ em values were red-shifted, and Φ em values ranged from 3 to 10%, which were in some cases enhanced relative to other BODIHYs with extended πconjugation reported to date. 61,63 The onset of absorption was used to estimate E g Opt using the equation E g Opt = 1240/ λ max onset (see Table 3 below). Relative to BODIHY 5c, which has an E g Opt of 2.45 eV, there was a noticeable decrease in E g Opt for compounds 8−13 ranging from 2.02 to 2.25 eV, with compound 12 possessing the lowest E g Opt . Given that BODIHY emission is diminished by the secondary inner-filter effect 61,63 relating to absorption/emission spectral overlap ( Figure S16), we also collected spectra for thin films spincoated from CH 2 Cl 2 solutions containing BODIHYs and poly(methyl methacrylate) (PMMA) in a 1:99 ratio (by weight). In doing so, we effectively weakened the reabsorption associated with tailing of the absorption spectra at low energies that overlap with the corresponding emission bands. The emission intensities for these films were enhanced and blueshifted relative to those observed for films of the pure compounds ( Figure S17).
Aggregation-Induced Emission. The AIE of BODIHYs 8−10, 12, and 13 was explored by preparing 25 μM solutions of varying H 2 O (f w ) fractions in THF and collecting their emission spectra (Figures 3 and S18−S20). Due to the limited solubility and instantaneous precipitation upon aggregate formation, the AIE behavior of BODIHY 11 was not further examined. An initial blue shift in λ em was observed when H 2 O was added to THF solutions of BODIHYs 8−10, 12, and 13 while the emission intensity remained low until f w reached 50% for 10 and 13, 70% for 12, and 75% for 8 and 9. Subsequent addition of H 2 O resulted in red-shifted λ em and increased emission intensities where maximum emission intensities were detected for when f w = 95% for 8 (Φ em = 4%), f w = 95% for 9 (Φ em = 2%), f w = 95% for 10 (Φ em = 4%), f w = 75% for 12 (Φ em = 3%), and f w = 85% for 13 (Φ em = 4%). The increased emission intensity is consistent with restriction of intramolecular motion (RIM) of the aryl substituents upon aggregation that was induced upon the addition of H 2 O. Enhancement factors were calculated from the ratio of maximum emission intensity and the emission intensity measured at f w = 0%. They were 14 for 8, 13 for 9, 12 for 10, 4 for 12, and 9 for 13. The observed red shift in λ em with increasing f w and the blue-shifted and enhanced emission observed for thin films containing PMMA doped with 1% BODIHY are consistent with trends observed for related BODIHYs 61−63 and imply that RIM is a plausible contributor to the AIE behavior observed.
Electronic Structure Investigations. The electronic structures of BODIHYs 5c and 8−13 were investigated computationally using density functional theory (DFT; LC-ωhPBE (ω = 0.14)/DGDZVP2) 69,70 (Figures 4, S21−S26, and  Table S1). TDDFT implicated the respective HOMOs, LUMOs, and LUMO + 1s as the orbitals contributing to the low-energy absorption bands for BODIHYs 5c and 8−13. In the cases of 5c and D−A BODIHYs 8−10, the HOMOs and LUMOs were delocalized π-type orbitals, suggesting that the low-energy transitions were of π → π* type. In A−D−A BODIHYs 11−13, the HOMOs are primarily localized on the π-spacers, while the LUMOs and LUMO + 1s were primarily located on the BODIHY units. This also suggests that electronic excitations for A−D−A BODIHYs 11−13 have charge-transfer characteristics due to the low-energy transitions arising from contributions from the HOMO → LUMO and HOMO → LUMO + 1 excitations (Figures S24−S26). The LUMOs of 11 and 13 were very similar; however, the LUMO for 12 is delocalized through the TH bridge. The differences in optical properties within this series of compounds directly related to the identity of the bridging donor. This statement was corroborated by the fact that the HOMOs spanned the bridging donor in each compound. Furthermore, although FLcontaining BODIHY 13 contains the largest π-system, it is the TH-containing BODIHY 12 that exhibited the lowest energy absorption maximum and thus the narrowest E g Opt . This  Solution spectra were collected for 5 μM analyte solutions in dry and degassed solvents. b The integrating sphere method was used to determine absolute quantum yields. c Thin films on quartz slides were spin-coated from CH 2 Cl 2 solutions. d The aggregate spectra were collected for the 25 μM THF/H 2 O mixtures of each compound that gave maximum emission intensities. observation was rationalized by comparing the angles between planes defined by the N-aryl substituents of the BODIHY heterocycle and the neighboring aryl ring of the bridging donor groups in the optimized structures ( Figure S27). In the case of 12, this angle was 18.8°, while in 11 (33.8°) and 13 (34.3°) the planarity, and thus the overall degree of π-conjugation, was disrupted due to the presence of two adjacent benzene rings that adopt a twisted geometry. 71,72 Electrochemical Properties. The redox properties of BODIHYs 8−13 were explored using cyclic voltammetry ( Figure 5 and Table 2), and all compounds exhibited reversible redox chemistry. As a point of reference, the parent BODIHY 5c was irreversibly oxidized at 0.77 V and reduced at −1.93 V relative to the ferrocene/ferrocenium (Fc/Fc + ) redox couple. 62 BODIHYs 8 and 10, which possess BE and FL substituents, respectively, each underwent reversible one-electron oxidation to produce radical cations at E ox1 = 0.69 and 0.62 V, respectively. TH-substituted BODIHY 9 exhibited irreversible oxidation waves, likely due to the reactive nature of terminal thiophene groups. 73,74 FL-substituted BODIHY 10 exhibited a second reversible one-electron oxidation at E ox2 = 0.97 V, indicating the formation of a dication. A−D−A BODIHY 11 includes a BE spacer and exhibited two overlapping oneelectron oxidations at 0.57 and 0.62 V, respectively. A−D−A BODIHYs 12 and 13, which include TH and FL donors, respectively, each exhibited two one-electron oxidations, with the former demonstrating the lowest oxidation potential of 0.38 V, indicating that TH-containing BODIHY 12 has the highest HOMO energy in this series. The stepwise oxidations for BODIHYs 9−13 and highly delocalized HOMOs in 8−10 indicated a degree of electronic communication throughout the π-systems. With the exception of BODIHY 13, which did not exhibit a reversible reduction, BODIHYs 8−12 exhibited reversible reduction waves between E red1 = −1.93 and −1.87 V, where the current response is doubled for 11 and 12, demonstrating that each BODIHY unit simultaneously underwent a one-electron reduction. Similar to other compounds containing two BODIHY fragments, 61,63 it can be concluded that the reductions were predominantly BODIHY based, given the similarity in reduction potentials and LUMO localization on the BODIHY fragment in compounds 8−13 (Figure 4). Compared to analogous BF 2 formazanate A−D−A compounds containing TH and FL as bridging donor groups, 60 (Table 2 and Figure 6). 75 The similar LUMO energies (−3.37 to −3.25 V) and localized LUMO densities of compounds 8−13 supported our conclusion that the reductions are BODIHY-based. The estimated HOMO energies ranged between −5.70 and −5.42 V and reflected the structural variation of the donor groups incorporated. The E g CV were estimated by taking the difference between E HOMO(CV) and E LUMO(CV) and range from 2.05 to 2.38 eV and are generally larger than those observed for related BF 2 formazanate A−D−A compounds. 76     was consistent with the E g Opt of 2.02 eV and the red-shifted λ max relative to other compounds described in this work. In all cases, strong agreement between the E g Opt and E g CV was noted, with minimal differences ranging from 0.03 to 0.19 eV (Table  3), and these band gaps were narrower than other BODIHYs reported. 66

■ CONCLUSIONS
In conclusion, we report the straightforward synthesis of D−A (8−10) and A−D−A (11−13) BODIHYs with PH, TH, and FL donor units using Stille cross-coupling chemistry. All compounds offered lower-energy absorption and emission bands compared to parent BODIHY 5c in solution and thin film, with weak emission observed in dilute solutions due to the rapid rotations and vibrations of appended rotatable substituents leading to nonradiative decay. Despite the fact that the emission intensity of BODIHYs was negatively affected by the secondary inner-filter effect, enhanced emission was observed in the solid state with Φ em values up to 10%. Compounds 8−10, 12, and 13 exhibited AIE, and the properties of thin films of PMMA containing 1% BODIHY corroborated that restriction of intramolecular motion was likely the origin of aggregate-and solid-state emission. The calculated frontier molecular orbitals of D−A compounds 8− 10 were highly delocalized while A−D−A analogues 11−13 possess HOMOs localized on the donor groups and LUMOs and LUMO + 1s localized on the BODIHY acceptors. Incorporating various donor groups led to the presence of multiple oxidation waves at potentials lower than those observed for related π-conjugated A−D−A materials containing BF 2 units. Finally, the E g CV and E g Opt determined for BODIHYs 8−13 agreed well, with TH-bridged BODIHY 12 exhibiting the lowest E g CV and E g Opt of 2.05 and 2.02 eV, respectively. The molecular materials described herein show the narrowest band gaps reported to date for BODIHY dyes and, combined with the thin-film emission properties of these materials, this could enable their use in light-harvesting applications which has been seen for other BODIHYs. 66 Collectively, these findings may inspire the design and development of BODIHYs to be further explored for optoelectronic applications such as organic electronics and photovoltaics.

■ EXPERIMENTAL SECTION
General Experimental Details. Unless otherwise stated, synthetic procedures were carried out under a N 2 atmosphere  6 ] as the supporting electrolyte and ∼1 mM analyte. The arrows denote the initial scan direction, and the dashed line is a voltammogram collected for BODIHY 9 for a wider potential window. The low current response observed for BODIHY 11 relates to its limited solubility.   6 ] as the supporting electrolyte. The scan rate was 0.25 V s −1 and potentials are reported relative to the Fc/Fc + redox couple. b Irreversible process, potential at maximum anodic current reported. c Two overlapping waves. d Irreversible process, potential at maximum cathodic current reported.

ACS Omega
http://pubs.acs.org/journal/acsodf Article using standard Schlenk techniques. Solvents were stored under a N 2 atmosphere over 4 Å molecular sieves after they were obtained from Caledon Laboratories, dried using an Innovative Technologies Inc. solvent purification system, and collected under vacuum. Reagents were purchased from Alfa Aesar, Fisher Scientific, Oakwood Chemicals, Sigma-Aldrich, or TCI America and used as received. 2-Bromo-9,9-dihexylfluorene, 77 2,7-bis(trimethylstannyl)-9,9-dihexylfluorene, 78 13 C{ 1 H} NMR spectra were referenced to CD 2 Cl 2 at 54.0 ppm. 11 B NMR spectra were reported relative to BF 3 ·OEt 2 at 0 ppm. 19 F NMR spectra were referenced to CFCl 3 at 0 ppm. 119 Sn NMR spectra were referenced to SnMe 4 at 0 ppm. Mass spectrometry studies were conducted using a Bruker MicrO-TOF II instrument or a Waters Synapt HDMS instrument using electrospray ionization (positive ion mode). UV−vis absorption spectra were collected using a Cary 5000 spectrophotometer between 250 and 800 nm. Extinction coefficients were determined from the slope of a plot of absorbance vs concentration including data for four separate sample concentrations. A Photon Technology International (PTI) QM-4 SE spectrofluorometer was used to collect emission spectra. The absorption maxima from the respective UV−vis absorption spectrum of each compound were used as the respective excitation wavelengths. Absolute emission quantum yields were measured using a Hamamatsu C11347-11 Quantaurus Absolute Photoluminescence Quantum Yield Spectrophotometer. Fourier transform infrared (FT-IR) spectra were recorded using a PerkinElmer Spectrum Two FT-IR spectrometer equipped with an attenuated total reflectance (ATR) attachment. Computational Methods. The Gaussian 16 software package 79 was used to perform molecular geometry optimizations and TDDFT calculations on a local machine or through the Graham cluster of Compute Canada using the DGDZVP2 basis set and the LC-ωhPBE 69,70 density functional with a tuned value of the range separation parameter ω = 0.14 and the polarizable continuum model (PCM) of implicit solvation (THF). Ground-state geometries were determined by exploring various initial conformations and choosing those with the lowest energy. The lowest energy structures for all compounds were explicitly confirmed by vibrational analysis to be true minima.
Thermal Analysis. BODIHYs were placed in a platinum pan and heated at a rate of 10°C min −1 from 10 to 1000°C under a flow of N 2 (60 mL min −1 ) using a TA Instruments Q50 thermogravimetric analysis instrument.
Preparation of Thin Films. The preparation of thin films of BODIHYs 8−13 required for absorption and emission spectroscopy was accomplished by filtering (PTFE membrane, 0.22 μm) ∼300 μL of 10 mg mL −1 solutions of each compound in CH 2 Cl 2 directly onto quartz slides that were previously dipped in CH 2 Cl 2 and air dried. The quartz slides were left at rest for 30 s before they were accelerated at a rate of 150−2000 rpm s −1 and spun for an additional 60 s. Thin films of PMMA doped with BODIHYs 8−13 were prepared similarly using a 10 mg mL −1 solution of 99:1 (by mass) PMMA/BODIHY. Aggregation Studies. Mixtures containing various ratios of degassed THF and deionized H 2 O (9.5 mL total) were combined with 0.5 mL of a 500 μM stock solution of the respective BODIHY in THF to yield 25 μM solutions with targeted H 2 O volume fractions (f w ). The solutions were mixed by three inversions before their immediate analysis.
Electrochemical Methods. Cyclic voltammograms were recorded for solutions of CH 2 Cl 2 containing the analyte (∼1 mM) and supporting electrolyte (0.1 M [nBu 4 N][PF 6 ]) using a Bioanalytical Systems Inc. (BASi) Epsilon potentiostat and analyzed using BASi Epsilon software. The electrochemical cells comprised of three electrodes, including a silver wire pseudo reference electrode, a glassy carbon working electrode, and a platinum wire counter electrode. Scan rates of 50−1000 mV s −1 were employed, and potentials were internally referenced against the Fc/Fc + redox couple (∼1 mM ferrocene as an internal standard) and corrected for internal cell resistance using the BASi Epsilon software.
Synthetic Procedures. 2-Trimethylstannyl-9,9-dihexylfluorene. 2-Bromo-9,9-dihexylfluorene (0.420 g, 1.02 mmol) was dissolved in THF (15 mL) before the reaction was cooled to −78°C and stirred for 10 min. nBuLi (0.450 mL, 1.12 mmol) was then added dropwise causing a color change from colorless to pale yellow while the reaction was stirred for 1 h at −78°C. In a separate flask, a solution of trimethyltinchloride (0.233 g, 1.17 mmol) in THF (2 mL) was prepared and added to the pale yellow lithium-containing solution in one portion. The reaction was then gradually warmed to room temperature and stirred for 16 h. The reaction mixture was then diluted with Et 2 O (20 mL), and the organic fraction was washed with H 2 O (15 mL), NaHCO 3 (2 × 15 mL), H 2 O (2 × 15 mL), and brine (15 mL); dried over MgSO 4 ; gravity filtered; and concentrated in vacuo to yield the crude product as a pale yellow solid that was used without further purification. Yield = 0.494 g, 98%. 1  General Stille Cross-Coupling Procedures for the Synthesis of D−A BODIHYs. Compound 7 (1 equiv), trialkyltin reagent (1 equiv), Pd 2 (dba) 3 (0.05 equiv), and P(o-tol) 3 (0.1 equiv) were added to an oven-dried 10 mL glass pressure tube equipped with a rubber septum under N 2 before dry, degassed toluene (∼5 mL) was added. The sealed tubes were then heated in an Anton Paar Monowave 50 reactor under the following conditions: (i) the temperature was ramped to 170°C over 8 min and (ii) the temperature was held at 170°C for 15 min with pressures reaching ∼4 bar during this time. Upon cooling to room temperature, the solutions were washed with 1 M KF (aq) (3 × 10 mL), dried over MgSO 4 , gravity filtered, and concentrated in vacuo to afford crude reaction products that were purified by column chromatography.